1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, an image to be transferred to a recording medium is represented on a plate, mat or other printing member as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the member is simply inked and the image transferred onto a recording material; the member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic in the sense of affinity for dampening (or "fountain") solution, and the necessary ink-repellency is provided by an initial application of such a solution to the plate prior to or during inking. The ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
A lithographic image is applied to a blank plate by altering its affinity characteristics in an imagewise pattern--that is, a pattern corresponding to the material to be printed. This may be accomplished photographically, by imagewise exposure of the blank plate to appropriate radiation followed by chemical development, or physically, using (for example) digitally controlled lasers to remove or facilitate mechanical removal of one or more plate layers in the imagewise pattern.
In a laser-based direct-write process, the laser imagewise removes (or facilitates removal of) ink-rejecting, non-image portions of the printing blank to reveal an ink-accepting layer that carries the image. In an indirect-write system, the laser instead removes ink-accepting portions of the blank. The choice of imaging mode depends less on the characteristics of the imaging system (since in digitally operated systems the mode can be changed merely by inverting the output bitmap) than on the structure of the printing member employed.
Lithographic printing members are now commonly imaged by low-power ablation imaging mechanisms. U.S. Pat. Nos. 5,339,737, 5,632,204, 5,783,364, and Reissue U.S. Pat. No. 35,512 (the entire disclosures of which are hereby incorporated by reference), for example, disclose a variety of ablation-type lithographic plate configurations for use with imaging apparatus that utilize diode lasers. For example, laser-imageable lithographic printing constructions in accordance with these patents may include a first, topmost layer chosen for its affinity for (or repulsion of) ink or a fluid to which ink will not adhere; an ablation layer, which volatilizes into gaseous and particulate debris in response to imaging (e.g., infrared, or "IR") radiation, thereunder; and beneath the imaging layer, a strong, durable substrate characterized by an affinity for (or repulsion of) ink or a fluid to which ink will not adhere opposite to that of the first layer. Ablation of the imaging layer weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step, creating an image spot having a lithographic affinity differing from that of the unexposed first layer.
During the imaging process or, subsequently, during use of the plate on a press, defects of electrostatic origin can be generated. These tend to occur around "floating" plate regions--that is, where an unimaged area is isolated from more extensive unimaged areas by a thin imaged boundary. A plate might, for example, consist of a layer of ink-rejecting silicone over a thin titanium imaging layer, which itself overlies an ink-receptive polyester substrate. The edges of the plate are generally pinned to a plate cylinder by metal clamps, which, due to their mechanical association with the press, are electrically grounded. Consequently, electrostatic charge accumulating on regions of the silicone held by the clamps dissipates or never develops. Islands of silicone within the plate, however, are electrically isolated from the clamps. As a result, the accumulated charge is trapped. The silicone and the polyester substrate are dielectric materials, so the potential difference between the charged silicone surface and the underlying metal plate cylinder (which, like the plate clamps, is at ground potential) can become considerable. If sufficient, the charge can arc across the imaged boundary to an unimaged region of silicone in contact with the plate clamps. The arc destroys a small portion of silicone, resulting in a print defect--that is, a spot that accepts ink although it was not imaged by the laser. These defects manifest themselves visibly on copy printed with the plate.